Throughout the world there is an increasing demand for premium fuels utilizable in transportation, with emphasis on fuels to supplement or supplant refined petroleum fuels. Alcohols represent the presently most promising of such premium fuels since alcohols can be produced effectively from renewable resources, biomass, particularly forest and farm wastes. In order to gain widespread usage and acceptance as a fuel for transportation needs, however, alcohol must be produceable at facilities that are net energy producers.
Biomass contains two basic constituents, carbohydrates and lignin. The carbohydrate content of the biomass consists of cellulose and hemicellulose, both polysaccharides. Both cellulose and hemicellulose may be converted to simple sugars, particularly hexose (including glucose, fructose, mannose and galactose) and pentose (including xylose and arabinose) sugars. The hexose sugars are conventionally fermented to form ethanol, and the pentose sugars are now fermentable utilizing a variety of commercially available microorganism strains, including (but not limited to) the yeast Pachysolen tannophilus NRRL Y-2460, the yeast Candida tropicalis ATCC 1369, Fursarium strains of fungus developed by Argone National Laboratory, and Bacillus Macerans developed by The University of California at Berkley, and Lawrence Berkley Laboratory. Ethanol, butanol, 2,3-butanediol, are typical alcohols produced. All are practical and versatile alcohols for transportation usages since high percentages can be mixed with gasoline without significant engine modifications, and they have relatively few corrosive effects on vehicle fuel systems.
Conventional techniques for producing alcohol center around the breakdown of cellulose into hexose and pentose sugars with subsequent fermentation of the hexose sugars. It is questionable, however, if there really is a net production of energy when cellulose hydrolysis is practiced since large amounts of energy are needed to maintain the high temperature and superatmospheric pressures needed for cellulose hydrolysis and since simple sugar solutions contain suspended lignin after cellulose hydrolysis, separation of such suspended lignin being very difficult and requiring expensive and energy intensive equipment.
According to the present invention, fuel alcohols can be produced from biomass containing carbohydrate and lignin with a net energy production. That is, according to the method of the present invention alcohol can be produced from biomass without the addition of energy from any external source, the introduced biomass itself providing both the raw material for the ethanol and the energy for all process steps. In fact, according to the present invention, depending upon the particular operational steps and parameters, more energy than is necessary for all of the process steps may be produced, and may be sold either as steam or electricity.
According to one aspect of the method of the present invention, alcohol is produced from biomass containing carbohydrate and lignin by particlizing and slurrying the biomass and then continuously subjecting the biomass to acid hydrolysis. The acid hydrolysis is performed at temperature, acid concentration, and residence time conditions sufficient to effect hydrolysis of the hemicellulose in the biomass to effect separation of pentose and hexose sugars therefrom into a hydrolysate having insufficient furfural to substantially inhibit fermentation microorganism growth, while not substantially hydrolyzing the cellulose in the biomass. Then fermentation of the pentose and hexose sugars in the hydrolysate is effected--such as by exposing them, under proper environmental conditions, to the yeast Pachysolen tannophilus NRRL Y-2460, Fursarium strains of fungus, and Bacillus Macerans, or the like--and then alcohol is produced from the fermented pentose and hexose sugars through normal processing (e.g. distillation]. By directing the hydrolysis toward hemicellulose breakdown, while not being concerned with cellulose breakdown, furfural production is minimized, as is energy usage, and the remaining biomass (including lignin and cellulose) remaining after acid hydrolysis of the hemicellulose can be burned to produce energy for all of the process steps, as well as additional energy for other purposes. It is necessary to minimize production of inhibitors, such as furfural, since small concentrations of some inhibitors can adversely affect the growth rate, or kill, the fermentation microorganisms.
The acid hydrolysis according to the present invention preferably is practiced with an acid concentration of about 2 to 10% by volume (preferably sulfuric acid), and the hydrolysis temperature is about 120.degree. C. or less. The residence time in the acid hydrolysis treatment is about 1 to 3 hours, and the treated biomass slurry has a biomass solids to liquid ratio of about 20/100 to 40/100 on a volume basis. The particle size of the biomass is about 1-4 mm.
Acid hydrolysis of biomass is practiced according to the present invention utilizing an upright vessel having a plurality of sets of apparatus, each apparatus set including a plurality of concentric non-rotatable screen members each having an apertured face and having a conduit leading away from the face to an area remote from the screen members; a plurality of vertically extending spray tubes disposed between the screen members; and means for moving the screen members alternately upwardly and downwardly. The apparatus sets are spaced vertically in the vessel and conduits extend from the screen members in an apparatus set so that they are operatively connected to the spray tubes in the next lower apparatus set. The conduits from the lowermost apparatus set lead exteriorly of the vessel to a further remote treatment point. Apparatus suitable for practicing acid hydrolysis according to the invention may be practiced utilizing apparatus such as shown in U.S. Pat. Nos. 3,372,087 or 4,172,037, the disclosures of which are hereby incorporated by reference herein. The method comprises the steps of continuously: Particlizing and slurrying the biomass. Flowing the slurry upwardly in the vessel past the apparatus sets. Moving the screen members of each set upwardly and downwardly, preferably being moved upwardly at a first velocity about equal to the velocity of the upwardly flowing slurry in the vessel and downwardly at a velocity much greater than the first velocity. Introducing hot washing liquid into the spray tubes of the uppermost apparatus set. Passing withdrawn liquid from each apparatus set screen members to the next lowermost apparatus set spray tubes. Introducing acid into the withdrawn liquid between one of the apparatus set screen members and the next lowermost set spray tubes. Withdrawing as hydrolysate the liquid withdrawn from the lowermost apparatus set screen members, and passing it on to a further remote treatment point; and withdrawing the biomass slurry from the top of the vessel and dewatering it.
By practicing the present invention a higher solids to liquids ratio can be utilized than is conventional, resulting in low acid usage and minimized energy expenditure for pumping and the like, and the pentose concentration in the hydrolysate is maximized.
According to another aspect of the present invention, a method of treating biomass having fermentable material is provided. The method comprises the steps of: Particlizing the biomass. Feeding the biomass into a surge bin, and steaming the biomass in the surge bin. Controlling the rate of feed of biomass from the surge bin to a conduit in which the biomass is slurried, and slurrying the biomass in the conduit. Refining the slurried biomass to effect reduction of the particle size thereof (preferably to about 1-4 mm.) and pumping the slurried, refined biomass to an upright acid hydrolysis vessel. Continuously effecting acid hydrolysis of the biomass in the vessel at a temperature of about 120.degree. C. or less to provide a hydrolysate. Passing a first portion of the hydrolysate to an ultimate destination for fermentation thereof, and passing a second portion of the hydrolysate to the slurrying conduit to effect slurrying of the biomass. Effecting washing of the biomass in the vessel after acid hydrolysis thereof by introducing a stream of hot wash water into a top portion of the vessel. Withdrawing hydrolyzed and washed biomass from the top of the vessel and effecting dewatering thereof; and passing the water from the dewatering of the biomass to the stream of wash water introduced into the vessel.
The hydrolysate preferably is acted upon by neutralizing it with lime, clarifying it, and then passing it to a conventional fermentation vessel. After fermentation, the "beer" is passed on to a yeast separation stage and then ultimately to conventional distillation towers where ethanol, butanol, 2,3-butanediol, and/or other alcohols, are produced. After dewatering the biomass is passed to a furnace, along with products from the yeast separation, to produce steam. The volume of steam produced is sufficient to supply the steam necessary for the distillation towers, for steaming of the biomass in the surge bin, for heating the wash water, and for running all necessary pumps, mixers, and the like. Additionally, there should be sufficient energy left over so that the entire facility is a net energy producer, producing steam or electricity in addition to the alcohol, without the introduction of energy from an external source (aside from the biomass itself).
Alternatively, instead of burning the biomass after dewatering, it may be passed to a cellulose hydrolysis stage wherein severe hydrolysis is practiced. The residue remaining from that stage may be burned as described above. Hydrolysate produced from either or both of the hemicellulose and/or cellulose hydrolysis stages may be subjected to acid recovery instead of neutralization, utilizing conventional techniques, before fermentation.
It is the primary object of the present invention to provide an efficient method of acid hydrolysis of biomass, primary for ultimate butranol, 2,3-butanediol, ethanol and like alcohol production. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.